In a conventional CATV transmission line monitoring system, transponders are mounted on an optical node, two-way amplifiers, power supplies, etc. installed on the transmission lines, and a transmission line monitoring computer in the center side periodically subjects each of the transponders to polling via a headend, so as to collect data of the devices to be monitored. In this monitoring system, each of the transponders directly monitors and operates an object device, therefore many parameters of, for example, opening and closing of a casing, power consumption, interior temperature, etc. can be collected. Furthermore, it has an advantage of having an advanced function such as a function of opening and closing an upstream gate of branching lines, in which, for example, part of an operation for specifying streamed noise failure in the upstream can be subjected to a remote operation performed at a headend station.
However, there have been various drawbacks, for example, since the conventional CATV transmission line monitoring system using transponders is an independent communication system in which both the hardware and software are dedicated to transmission line monitoring, the system is expensive, and the transponders are mounted in outdoor-use device casings, accordingly, harsh environmental conditions, i.e., high temperature and high humidity cause failure and malfunctions in the transponders per se, thereby generating operation/maintenance cost increasing factors.
Therefore, attention has been focused on cable modems used in the cable television internet, and it is proposed a simple CATV transmission line monitoring system for monitoring transmission lines by utilizing standard network management protocol of cable modems, as a simple CATV transmission line monitoring system in which excessive monitoring and control functions provided in the case when transponders are used are omitted and merely minimum functions required for maintenance of transmission lines are implemented (JP2002-247607).
Meanwhile in a conventional system for monitoring transmission lines by utilizing cable modems, measured values according to cable modems, such as downstream reception levels and downstream S/N, are collected through polling, and failure is detected by comparing the measured values with predetermined threshold values. However, since CATV transmission lines are strongly affected by temperature variation due to, for example, solar irradiation and seasonal variation, failure determination using threshold values as absolute values is not always appropriate. Therefore, relative evaluation in which measured values collected through polling are stored as statistical data in, for example, year units, and threshold values are set relatively with the average value of the corresponding season and time of the previous year serving as a reference, thereby effectively determining failure. However, measured values of at least one year have to be stored for such relative evaluation, and it takes too much time to start the relative evaluation. Moreover, the diurnal temperature variation of the same day of the previous year is not necessarily same as the actual diurnal temperature variation. Therefore, threshold values are set relatively using the monthly average value of the same month of the previous year as a reference, wherein it is expected that the difference with the actual diurnal temperature variation is considerably large when the monthly average is used, and, even in failure determination according to relative threshold values, it is difficult to sufficiently eliminate the influence due to temperature variation.
When failure is determined from measured values obtained through polling of cable modems disposed in the premises of subscribers on feeder lines which are ends of CATV transmission lines, generally, failure points are mapped with the positions of the cable modems positioned at ends in the system diagram of the CATV transmission lines having a tree structure, and geometry analysis is performed, thereby estimating failure locations on the transmission lines. If there are not many failure points, the estimation of malfunction locations is carried out comparatively easily. However, because measured values are collected by subjecting a plurality of cable modems, which are disposed in the premises of subscribers on end feeder lines of CATV transmission lines, to polling, so as to determine malfunctions, in some cases, measured values of a plurality of cable modems are determined to be in failure at the same time, and the substances of the failure are different in failure points. In this case, analysis of failure locations by means of geometry analysis cannot be assumed to be always appropriate, and an algorithm of which process for estimating failure locations that is simple and highly accurate is required.
In addition to the cable modems disposed in the premises of subscribers on end feeder lines of CATV transmission lines, cable modems are disposed also on trunk lines in accordance with needs, and measured values are collected through polling so as to determine failure. However, measured values of the cable modems disposed on transmission lines in this manner are sometimes erroneously determined to be in failure due to causes unique to cable modems even though upstream and downstream signals are normal, and presence of erroneous failure points on the transmission lines may lead to erroneous estimation of malfunction locations.
Furthermore, measured values collected through polling from cable modems disposed on premises of subscribers on feeder lines and a trunk line of CATV transmission lines are stored, and subjected to multipoint statistical display for carrying out comparison display of, for example, downstream signal levels of a plurality of cable modems in a graph of monthly variation thereof. However, the number and types of transmission devices to be passed through from a headend to cable modems vary in each cable modem, and there is a problem that, even if the measured values of such cable modems are subjected to multipoint statistical display, appropriate comparison and determination cannot be carried out since the correlation as a transmission system is low.